Elimination from platinum complexes

This reaction, like dicarbene recombination, also has its analog in coordination chemistry, that is, reductive elimination of tetramethylene and pentamethylene ligands from platinum complexes yields cyclobutane and cyclopentane, respectively (777). According to this direct ring closure mechanism, the observed selectivity for dehydrocyclization of n-alkanes on metals (nonformation of quaternary-secondary and tertiary-secondary C-C bonds in reactions of type A and B) should be interpreted in terms of simple steric effects. However, although, in the case of platinum, the concepts of steric hindrance could account for the change of selectivity that occurs with decreasing metal particle size (i.e., cyclization of n-hexane takes place on... 

Reductive elimination from octahedral complexes of rhodium(III) and irid-ium(III) has been examined mainly for C-H bond formation. While some of the complexes undergo a dissociative mechanism similarly to platinum(IV) and palla-dium(IV) analogs, direct reductive elimination without preliminary ligand loss has also been documented. 

Reductive eliminations to form C-C bonds from platinum complexes also include those containing both Pt(II) and Pt(IV) centers. Reductive elimination of biaryls and cyclopropanes from Pt(II) complexes were shown in Equations 8.39-8.41. Reductive eliminations from Pt(IV) were shown in Equations 8.35 and 8.37. ... 

Silyl(pinacol)borane (88) also adds to terminal alkenes in the presence of a coordinate unsaturated platinum complex (Scheme 1-31) [132]. The reaction selectively provides 1,2-adducts (97) for vinylarenes, but aliphatic alkenes are accompanied by some 1,1-adducts (98). The formation of two products can be rationalized by the mechanism proceeding through the insertion of alkene into the B-Pt bond giving 99 or 100. The reductive elimination of 97 occurs very smoothly, but a fast P-hydride elimination from the secondary alkyl-platinum species (100) leads to isomerization to the terminal carbon. 

There are now a number of quite stable Pt(IV) alkyl hydride complexes known and the synthesis and characterization of many of these complexes were covered in a 2001 review on platinum(IV) hydride chemistry (69). These six-coordinate Pt(IV) complexes have one feature in common a ligand set wherein none of the ligands can easily dissociate from the metal. Thus it would appear that prevention of access to a five-coordinate Pt(IV) species contributes to the stability of Pt(IV) alkyl hydrides. The availability of Pt(IV) alkyl hydrides has recently allowed detailed studies of C-H reductive elimination from Pt(IV) to be carried out. These studies, as described below, also provide important insight into the mechanism of oxidative addition of C-H bonds to Pt(II). 

If alkyl groups having (3-hydrogens are present on platinum cis to an open site, (3-H-elimination will indeed occur, reversibly sometimes, and it can occur both from Pt(II) and Pt(IV) (52,97,213-219). Catalytic dehydrogenation of an alkane using a soluble platinum complex has been reported in an early study on acceptorless thermal dehydrogenation. At 151 °C, cyclooctane was catalytically dehydrogenated (up to 10 turnovers)... 

Hydroxylamine plays the same part in the molecule as ammonia in ammino-platinum compounds, but the substances differ somewhat in chemical behaviour, for hydroxylamine is more readily eliminated than ammonia from the complex. Also, m-dihydroxylamino-dichloro-platinum is not obtained by the interaction of free hydroxylamine and potassium chloroplatinite, the method used for the preparation of cis-dichloro-diammino-platinum. Again, tetrammino-platinous hydroxide, [Pt(NH3)4](OH)2, is a very strong base and easily soluble in water, whilst tetrahydroxylamino-platinous hydroxide, [Pt(NH2OH)4](OH)2, is almost insoluble in water and a comparatively weak base.2 For this reason Werner 3 suggested a different formula for the two substances, and indicated that possibly in the tetrahydroxylamino-com-pound the co-ordination number of the metal is six and not four, as in the tetrammino-compounds thus ... 

Oxidative addition of the Si-aryl carbon bond in the silacyclobutene ring to Pt gives the optically active intermediate Pt-complex. Further coordination of (+)-l-methyl-l-(l-naphthyl)-2,3-benzosilacyclobut-2-ene to the complex and cr-bond metathesis will provide the cyclic dimer Pt-complex. Reductive elimination from the intermediate platinum complex gives cyclic polymers and oligomers. Preference of cr-bond metathesis over reductive elimination gives polymers of higher molecular weight. The presence of EtsSiH in the system results in the formation of linear products via cr-bond metathesis. 

Stable zirconium, platinum, molybdenum, and tungsten complexes of cyclooctyne, a zirconium complex of cydoocta-5-enyne, and a bimetallic molybdenum complex of cyclocta-3,7-dienyne have been discussed in earlier reviews.28 More recently, two stable zirconocene complexes of cycloocta-trienyne (275 and 276) have been prepared101 by /3-hydride elimination from 274 in the presence of PMe2R [Eq. (45)]. 

In 1-substituted pyrimidine complexes, migration of platinum from en-docyclic to exocyclic nitrogen has been observed, i.e., migration of PtIV from N(3) to the exocyclic C(4)-NH2 group in 1 -MeCyt (1 -MeCyt = 1 -meth-ylcytosine) [36]. In the initial complex trans, trans, trans-[Pt(OH)2(NH3)2(l-MeCyt-N(3))2]2+, the V(3),A(4 )-chelate is formed with the elimination of H20 from the complex (Scheme 7). Addition of H20 to the chelated com-... 

R R SiH(CH=CH2) (B) yields an 18 VE species, which rearranges from tt to o complex by hydrogen migration from platinum to the coordinated olefin and Pt-C bond formation (C). The vacant coordination position that forms is occupied by an external Cl ligand. Reductive elimination of the carbosilane and regeneration of [PtCy complete the cycle. 

Although it is now almost fifty years since Speier and his colleagues first announced the chloroplatinic acid-catalyzed hydrosilation of olefins, we are still far from complete control of the chemistry. A particular problem is the suppression of double bond migration. A solution of this problem will require a more detailed understanding of the factors affecting the relative rates of P-hydride elimination from an alkyl group and of the reductive elimination of Si-H from a platinum silyl hydride complex. Another factor which is poorly understood is suppression of the irreversible reduction of the platinum catalyst to Pt° metal. Both of these problems can greatly increase costs of production of certain products. 

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